Since we have moved out of the era of brick phones and indestructible Nokias, and have entered the world of fragile iPhones, the market for good phone cases has widened greatly. Cases used to be just stylistic choice. Now they are almost necessary since you are investing hundreds of dollars into an iPhone (they are totally worth it though). How does the case work to prevent damage to your phone though?
When you drop your phone on a surface, the surface will act with an equal and opposite force on the iPhone. A large enough force is able to dent or shatter a phone, if dropped without any protection. Cases are built so that, on impact, the forces are distributed evenly across the phone. The materials used are also able to absorb some of the forces, again lessening the force that is exerted on the phone itself.